1. Field of the Invention
The present invention relates in general to hydraulic pumps used in motor vehicles, and more particularly to vane pumps of a type which is used as a hydraulic power source for a power steering device or the like.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional vane pump of the above-mentioned type will be described, which is disclosed in Japanese Patent First Provisional Publication No. 63-167089. The vane pump of the publication comprises generally a side plate, a rear cover, a rotor carrying a plurality of projectable vanes, and a cam ring accommodating the rotor and slidably disposed between the side plate and the rear cover.
FIG. 7 of the accompanying drawings is a plan view of the rear cover used in the known vane pump of the publication. The inner surface of the rear cover to which the rotor slidably contacts is designated by reference "a". As shown, the inner surface "a" is formed with two pairs of back pressure grooves "b" and "c" through which a discharged fluid is led into vane mounting grooves of the rotor for radially outwardly biasing each vane to contact against an oval inner surface of the cam ring. The two pairs of the back pressure grooves "b" and "c" are arranged symmetrical with respect to a rotation center of the rotor so as to deal with the suction and discharge states of pump chambers of the pump respectively. Each pump chamber is defined by neighboring two vanes. The back pressure grooves "b" are in communication with a discharge chamber of the pump. A throttle groove "e" is formed on the inner surface "a", which extends between each pair of the back pressure grooves "b" and "c". More specifically, the throttle groove "e" is arranged to communicate the vane mounting grooves.
When a vane travels in a suction zone of the pump, the mounting groove for the vane tends to increase its volume. Under this condition, the discharge fluid pressure is applied to the vane mounting groove through the back pressure grooves "b" for assuring the contact of a head of the vane against the oval inner surface of the cam ring. When, on the other hand, the vane travels in a discharge zone of the pump, the vane mounting groove tends to reduce its volume. Under this condition, a part of the hydraulic fluid in the vane mounting groove is forced to return from the back pressure groove "c" to the other back pressure groove "b" through the throttle groove "e".
Since the shape of each throttle groove "e" has a direct effect on the tracing ability of the vane head against the oval inner surface of the cam ring, the throttle grooves "e" have been machined with a high accuracy.
Also the side plate has back pressure grooves and throttle grooves which are similar in construction to those of the above-mentioned rear cover.
However, due to its inherent construction, the vane pump of the above-mentioned type has the following drawbacks.
That is, usually, the rear cover and the side plate are produced by casting aluminium alloy or the like whose hardness is less than that of the rotor. Thus, the inner surface of the rear cover and that of the side plate tend to be markedly worn away by the rotor. This means that the throttle grooves "e" tend to be deformed in a short period of time, having a bad effect on the performance of the pump. Furthermore, machining the rear cover and the side plate for forming the accurately dimensioned throttle grooves requires the employment of skilled labor and time.